There have been various technological approaches to produce a writing pad as a replacement of paper and pencil or chalk on slate. The best known examples are toys. The ETCH-A-SKETCH™, introduced in the 1960s, is one such device. In this device, a movable stylus removes a powder material from inside a screen to make a dark line. The image is erased by turning the device upside down and shaking it to smooth out the surface. Another famous example is the MAGNA DOODLE™, which is a magnetophoretic device in which a stylus with a magnet on the tip is used as the pen to draw a line. The device is erased with a thin long magnet behind the screen. Over 40 million of these devices have been reportedly sold.
Other writing pads have also been proposed. U.S. Pat. No. 4,525,032 to Hilsum is one such example where cholesteric or a smectic liquid crystal is used to provide a semi-permanent record of the path traced by a stylus on a display and used as a re-usable writing pad. According to Hilsum a layer of a liquid crystal material is contained between two substrates. A stylus having a tip contacts the front substrate and changes the state of selected areas of the liquid crystal layer at positions adjacent the pen tip to provide observable information corresponding to the pen movement. The pen may have a pointed tip, a heated tip, a light emitting tip, or a tip connected to a high voltage high impedance source. At least one substrate of the display can be deformable, thin, or flexible so that the liquid crystal layer may be changed from one state to another by localized application of pressure, heat, light, electrostatic charge, or an electric field. The resultant image on the display is erased by deformation of the layer, e.g. flexing, heating and cooling, or by an electrical field.
A practical problem with the Hilsum device is erasing the image. It is slow and inconvenient to heat or flex the device to erase the image. Hilsum discloses an electronic means of erasure using a special cholesteric liquid crystal in which the frequency of an AC field is applied to the stylus or electrodes. The frequency is changed to enable a writing state or an erasure state. However, this is not without problems in that crossover frequency between writing and erasing is strongly temperature dependent and the frequencies as well as the voltages are very high, consuming a lot of power causing very limited battery lifetime.
A considerable improvement was made with the discovery of bistable cholesteric liquid crystals (see U.S. Pat. No. 5,453,863). Cholesteric liquid crystalline materials are unique in their optical and electro-optical features. These materials possess a helical structure in which the liquid crystal (LC) director twists around a helical axis. The reflected light is circularly polarized with the same handedness as the helical structure of the LC. They can be tailored to Bragg reflect light at a pre-selected wavelength and bandwidth by controlling the pitch of the helical twist through the concentration of chiral dopants and the birefringence of the nematic host, respectively. If the incident light is not polarized, it will be decomposed into two circular polarized components with opposite handedness and one of the components reflected.
The cholesteric material is typically electrically switched to either one of two stable textures; planar or focal conic as described, for example, in the U.S. Pat. No. 5,453,863. In the planar texture, the director of the LC (direction of the long axis of the molecule) is uniformly parallel to the plane of the substrates across the cell but has a helical twist perpendicular to the plane of the substrates. It is the helical twist of the uniform planar texture that Bragg reflects light in a selected wavelength band. The focal conic texture contains defects that perturb the orientation of the liquid crystalline helices. In the typical focal conic texture, the defect density is high; thus the helical domain size becomes small and randomized in orientation such that it is just weakly scattering and does not reflect impinging light (i.e., it is essentially transparent to incident light). Once the defect structures are created, they are topologically stable and cannot be removed unless by some external force such as an electric field or melting the material out of the liquid crystalline phase to the isotropic. Thus, the focal conic texture remains stable and forward scatters light of all wavelengths into an absorbing (usually black) background. These bistable structures can be electronically switched between each other at rapid rates (on the order of milliseconds). Gray scale is also available within a single pixel through various switching schemes in order to adjust the density of reflective helical domains that are oriented perpendicular to the substrates (planar texture) to the randomized forward scattering domains (focal conic texture).
Bistable cholesteric liquid crystal displays have several electronic drive features that other bistable reflective technologies do not. For addressing a matrix of many pixels in a display, the characteristic of a voltage threshold is used. A threshold is used for multiplexing a row/column matrix without the need of an expensive active matrix (transistor at each pixel). Bistability with a voltage threshold allows very high-resolution displays to be produced with low-cost passive matrix technology. Gray scale capability allows stacked RGB, high-resolution displays with full-color capability where as many as 4096 colors have been demonstrated.
In a cholesteric liquid crystal display, the liquid crystal is typically sandwiched between two substrates that are spaced to a particular gap. The substrates can be either glass or plastic. The bottom substrate is painted with a light absorbing (black or colored) background. The cell gap is usually set by plastic or glass spacers that are either cylindrical or spherical in shape. In most cholesteric liquid crystal displays, the cell gap is not intentionally changed. If one presses on the top substrate of the cholesteric LCD, the liquid crystal can be displaced (since fluids are not very compressible) and induced to flow radially out of the area. Of principle interest is that when the focal conic texture of the cholesteric liquid crystal is induced to flow, the resulting texture is the planar state. The reflective planar state contrasts well to the dark focal conic background. This is a principle behind U.S. Pat. No. 6,104,448 “Pressure Sensitive Liquid Crystalline Light Modulating Device and Material,” incorporated herein by reference in its entirety, which discloses that application of a mechanical stress to the liquid crystalline light modulating material changes an initial light scattering focal conic texture to the light reflecting planar texture. The U.S. Pat. No. 6,104,448 discloses a polymer network that is soluble with the chiral nematic liquid crystal and phase separates to form separated polymer domains that stabilize the thickness of the cell structure.
In the U.S. Pat. No. 6,104,448, an image can be written on the device with an untethered stylus or finger. The entire image is erased with the push of a button that applies a low voltage DC pulse to the cholesteric device. Other advantages of bistable cholesteric materials is that an image created on the writing pad display does not degrade with time and lasts indefinitely without application of an electric field, until erased. The time for erasing the entire image is less than a second, making the bistable cholesteric liquid crystal display a practical device for a writing pad. However, the device of the U.S. Pat. No. 6,104,448 completely erases the entire writing surface and does not erase selected portions of the image.
It would be desirable to have an electronic writing device whereby a selected portion of the image can be erased while retaining the remainder of the image. A pencil with a rubber eraser on paper and chalk with a cloth eraser on a chalk board have been unmatched in convenience by providing selectable erase. Likewise, a desirable feature of an electronic writing pad is selected erasure of the image accomplished with an untethered instrument such as a stylus.